Plasma spraying is a powerful technique used widely to produce protective coatings on a large variety of substrates. For example, thermal barrier coatings are plasma sprayed in producing aircraft engines, and ceramic and metal coatings are plasma sprayed for various purposes. Coating properties depend upon many spraying parameters, some of them being related to the spray gun operation. Consequently spraying process control has been implemented by monitoring and regulating such gun input variables as arc current and power, arc gas flow rates, powder feed rate, and powder carrier gas pressure, to keep them at predetermined optimum values. This control approach has been found to be complex because a large number of interrelated input variables must be monitored, and has been found to be incomplete because some variables, such as electrode wear state, cannot be monitored at all.